1. Field of the Invention
The present invention relates to an apparatus and a method for polishing a material such as a semiconductor wafer, and a method of conditioning a surface state of a polishing pad used for polishing the material to be polished.
This specification is on the basis of Japanese Patent Applications (Japanese Unexamined Patent Application Publication No. 11-244120, No. 2000-004058, No. 2000-093834, and No. 2000-099648), and the disclosures of these Japanese Patent Applications are incorporated herein as a part of this specification by reference.
2. Description of the Related Art
A semiconductor wafer cut out of a silicon ingot, for example, from which a semiconductor device is formed, must be polished for realizing fine device structures, in such a manner that the wafer surface acquires a mirror surface free from defects and having high accuracy. A chemical mechanical polishing process (CMP process) capable of providing a high degree of planarity has been employed to polish surfaces of materials for which high polishing accuracy is required, including semiconductor wafers and wafers having devices formed thereon (hereinafter referred to simply as wafers).
The CMP process chemically and mechanically polishes and flattens surfaces of materials to be polished by employing, for example, an alkaline slurry using SiO2, a neutral slurry using CeO2 or an acid slurry using Al2O3.
A known polishing apparatus 1 for polishing a wafer surface using a CMP process is schematically shown in FIG. 23 which is an enlarged perspective view of principal part of the apparatus. As shown, a polishing pad 4 made of hard urethane, for example, is affixed to a disk-shaped platen 3 attached to a central shaft 2, and a polishing head 5 holding a wafer W is from the central shaft 2 of the platen 3. The polishing apparatus 1 polishes one surface of the wafer W by moving the polishing pad 4 and the wafer W relative to each other, while the polishing head 5 holds the wafer W in contact with the surface of the polishing pad 4, and a slurry S is supplied between the wafer W and the polishing pad 4.
Another known example of a apparatus for polishing a wafer surface with the CMP process is shown in FIG. 24. In the following description, components in FIG. 24 having essentially the same construction as those of the polishing apparatus 1 shown in FIG. 23 are denoted by the same reference numerals. Referring to FIG. 24, a polishing apparatus 10 comprises a polishing head 5 holding a wafer W to be polished, and a polishing pad 4 affixed to an upper surface of a disk-shaped platen 3. A plurality of polishing heads 5 are attached to the underside of a carousel 11 serving as a head driving mechanism. The polishing heads 5 are each supported by a spindle 16 to rotate on the polishing pad 4 in planetary motion. In this connection, the center of the platen 3 may be positioned offset from the center about which the polishing heads 5 revolve.
The platen 3 is horizontally disposed at the center of a base 12 and rotated about its own axis by a platen driving mechanism (not shown) provided within the base 12. Posts 13 are vertically provided on both side portions of the base 12, and an upper attachment plate 14 is disposed between the posts 13 to support a carousel driving mechanism 15. The carousel driving mechanism 15 has the function of rotating the carousel 11 provided below the carousel driving mechanism 15 about its own axis.
Abutment portions 17 are disposed on the base 12 to project upward therefrom, and a gap adjusting mechanism 18 is provided at a top of each of the abutment portions 17. Above the abutment portions 17, engaging portions 19 are provided in one-to-one opposite relation. The engaging portions 19 are fixed to the upper attachment plate 14 and project downward from the upper attachment plate 14. By regulating the gap adjusting mechanisms 18 and bringing the engaging portions 19 into contact with the abutment portions 17, the distance between the polishing head 5 and the polishing pad 4 is set to an appropriate value. The wafers W are then polished by rotating the carousel 11, the polishing heads 5 and the platen 3 while the wafers W held by the polishing heads 5 are kept in contact with the surface of the polishing pad 4.
In any of such polishing apparatuses employing the CMP process, properties of the polishing pad and the slurry, polishing time, etc. are changed depending on the wafer polishing conditions, to adjust those parameters so that wafers are polished under the optimum conditions. Here, the term xe2x80x9cproperties of the slurryxe2x80x9d means the material characteristics and size of abrasives forming the slurry, properties of a solution (such as pH and concentration of the solution) in which the abrasives are suspended, etc.
Furthermore, in the polishing apparatuses described above, a step of polishing a wafer surface is divided into several stages, for example a rough polishing stage for eliminating roughness on the wafer surface, a fine polishing stage for flattening the wafer surface, and a finish polishing stage for finishing the wafer surface to a mirror surface. In each stage, the wafer is polished under the respective optimum conditions.
In addition, thin films such as an oxide film and a metal film are formed on a wafer surface to form a circuit on the wafer. Therefore, the polishing conditions are also changed depending on material characteristics of the thin films formed on the wafer surface.
To enable a wafer to be polished successively under plural sets of different polishing conditions, there is known a polishing apparatus comprising a plurality of polishing stations, each of which includes a platen provided with a polishing pad and a driving device for driving the platen for rotation.
In such a polishing apparatus designed to move a wafer between the polishing stations operated under the different polishing conditions, if the slurry adhering to the wafer and the polishing head is not completely removed, the polishing station to which the wafer is transferred from a preceding one is contaminated with the slurry adhering to the wafer and the polishing head, because the properties of the slurry, i.e., the material characteristics and size of abrasives, properties of a solution, etc., differ between the polishing stations. Such mixing of slurries having different properties not only changes the polishing conditions in the polishing station of subsequent stage, but also may bring the slurry into a cohered or gelled state, thereby damaging the wafer surface, or may cause abnormal corrosion of the wafer.
For preventing such drawbacks, a cleaning device has been hitherto provided between the polishing stations to clean the wafer and the polishing head when the wafer and the polishing head are moved from one polishing station to another. However, the cleaning is usually carried out with the wafer kept supported by the polishing head, and the slurry having intruded into, e.g., between the wafer and the polishing head, cannot be completely removed. The remaining slurry is therefore caused to flow out and mix with a new slurry while the wafer is polished in the subsequent polishing station. As a result, the above-described drawbacks cannot be avoided using the present methods.
Further, in any of the polishing apparatuses described above, a number of fine holes, grooves or the like are formed in the polishing pad 4 to hold a slurry, and a wafer is polished with the slurry held on the surface of the polishing pad 4 with the aid of those holes, grooves or the like. However, repeating the wafer polishing gives rise to a problem of reducing the polishing accuracy and the polishing efficiency of wafers, because the slurry and other foreign matter (such as chips or debris particles of the polishing pad 4 and the wafer) adhere to the surface of the polishing pad 4, and the polishing pad 4 is loaded with them. Also, there is a risk that properties of the foreign matter may be changed with the lapse of time and may cause scratches in the wafer.
In view of the above problems, a conditioner 21 (loading eliminating device) shown in FIG. 23, for example, has been hitherto provided in the polishing apparatus to separate the foreign matter deposited on the polishing pad 4 from it and to prevent the polishing pad 4 from being loaded with the foreign matter (this operation is called conditioning). Additionally, though not shown, the conditioner 21 is also provided in the polishing apparatus 10 shown in FIG. 24.
As seen from FIG. 23, which is an enlarged perspective view of a principal part of the polishing apparatus, the conditioner 21 comprises a disk-shaped dresser 22 having a dressing portion (not shown in FIG. 23) formed on one surface, a rotating/driving mechanism (not shown) for rotating the dresser 22 about its own axis, and an arm 23 for holding the dresser 22. The conditioner 21 further comprises a moving mechanism 24 for moving the dresser 22, which has been carried onto the polishing pad 4, parallel to the polishing pad 4 while keeping the dresser 22 in a state where a surface of the dressing portion of the dresser 22 is contacted with the polishing pad 4 and the dresser 22 is pressed against the polishing pad 4. Instead of being positively rotated with the rotating/driving mechanism as described above, the dresser 22 may be constructed so as to rotate by frictional forces developed between the dresser 22 and the rotating polishing pad 4 during the conditioning.
The moving mechanism 24 comprises, for example, an X-axis rotating/driving mechanism 25 for rotating the arm 23 about an X axis (substantially perpendicular to the surface of the polishing pad 4), and a Y-axis rotating/driving mechanism 26 for rotating the arm 23 about a Y axis (substantially parallel to the surface of the polishing pad 4). Also, the moving mechanism 24 further comprises, though not shown, a mechanism for adjusting an inclination angle of the dresser 22 relative to the arm 23 so that the dresser 22 is held parallel to the polishing pad 4 when the dressing portion surface of the dresser 22 is brought into pressure contact with the polishing pad 4.
During the conditioning of the polishing pad 4, the conditioner 21 shown in FIG. 23, by way of example, operates as follows. First, the dressing portion surface of the dresser 22 is brought into pressure contact with the polishing pad 4 by the moving mechanism 24. Then the dresser 22 is rotated about its own axis, and at the same time it is reciprocally swung on the rotating polishing pad 4 by the X-axis rotating/driving mechanism 25. In this way, the dressing portion of the dresser 22 slightly chips off the surface of the polishing pad 4 to separate foreign matter deposited on the surface of the polishing pad 4. The polishing pad 4 is thereby given a fresh surface free from loading and dulling to restore the polishing ability of the polishing pad 4 (to achieve the so-called dressing).
In addition to the above-described conditioner 21, there are also known other loading eliminating devices, for example one wherein a cleaning liquid is sprayed under high pressure toward the polishing pad 4 by a cleaning-liquid jetting means to wash foreign matter out of the surface of the polishing pad 4, and wherein the foreign matter is beaten and scrubbed out of the surface of the polishing pad 4 with a rotating brush.
Even after use of the loading eliminating device, if foreign matter still remains on the surface of the polishing pad 4, it may cause scratches, etc. in the wafer being polished by the polishing pad 4 or may vary the polishing rate. To avoid the above problem, in the conventional loading eliminating device, the slurry and other foreign matter (including abrasives scraped off the dresser 22), which have been separated from the polishing pad 4, are removed from the polishing pad 4, for example by supplying a cleaning liquid (including pure water), a slurry, etc. to the surface of the polishing pad 4 so as to wash out the remaining foreign matter. However, the amount of the supplied cleaning liquid or slurry is not large, and hence the removal efficiency of the foreign matter is low.
In a method of washing out the foreign matter with a slurry, the slurry supplied for the washing-out cannot be used to polish a wafer because of a fear that the foreign matter can remain mixed in the slurry. Also, even in a method of washing out the foreign matter with a cleaning liquid (e.g., pure water), a part of the cleaning liquid remains on the polishing pad 4, and therefore the concentration, pH, etc. of a slurry newly supplied to the surface of the polishing pad 4 for polishing the wafer are changed, whereby the wafer polishing conditions are also changed. Thus, the above-described conventional methods are disadvantageous because the slurry or cleaning liquid remaining on the polishing pad 4 must be replaced by the newly supplied slurry for polishing the wafer, and so the amount of the slurry used and the cost are increased.
Further, because the conventional loading eliminating device employs a rotating dresser or brush, a cleaning liquid sprayed under high pressure, or the like, there has been a risk that the foreign matter may scatter to the surroundings and remain on the polishing pad 4, or may adhere to the polishing apparatus 1 and then drop off onto the polishing pad 4 from the polishing apparatus 1, thereby contaminating the polishing pad 4. In addition, there has been a risk that the scattered foreign matter may intrude into moving parts of the polishing apparatus 1 and eventually impede the normal operation of the polishing apparatus 1.
Further, in the conditioner 21 shown in FIG. 23, by way of example, materials such as chips of the polishing pad 4, debris particles of the wafer and the slurry having the changed properties, for example, are deposited primarily on the dressing portion surface of the dresser. If those deposits are solidified or gelled to have their properties changed, the deposits having the changed properties are scaled off and drop onto the polishing pad 4 when the polishing pad 4 is subjected to conditioning. This has raised a problem that the dropped scales damage the wafer surface, to produce scratches, etc. during the subsequent wafer polishing. Also, even when the dressing portion surface of the dresser is immersed in a cleaning liquid, the deposits cannot be removed from the dressing portion surface because of sticky adhesion to it, and a problem similar to that described above has been caused.
In any of the above-described polishing apparatus, grooves are formed beforehand in the surface of the polishing pad 4 to introduce air between the wafer and the polishing pad 4, so that the wafer will not adhere to the surface of the polishing pad 4 and will not be left on the polishing pad 4 when the polishing head 5 is elevated after completion of the wafer polishing. The presence of such grooves prevents the wafer from adhering to the polishing pad 4 and ensures proper progress of the polishing step.
The grooves each have a depth of about 0.5 mm. In addition to the above effect, the grooves also hold the slurry which is used for the wafer polishing on the polishing pad 4, and receive the chips, debris particles, etc. generated during the wafer polishing, thereby preventing the occurrence of scratches in the wafer surface.
In this connection, the surface of the polishing pad 4 is usually conditioned for each cycle of polishing so that the polishing ability is adjusted to fall within the appropriate range, as described above. The conditioning of the polishing pad 4 is performed, for example, by chipping off the surface of the polishing pad 4 in a thickness of about 1-2 xcexcm by the dresser 22 of the conditioner 21 and forming a fresh surface of appropriate roughness.
Because the surface of the polishing pad 4 is chipped off bit by bit whenever it is subjected to the conditioning, the depth of the grooves formed in the surface of the polishing pad 4 is gradually reduced. The shallower the grooves, the less effective is the polishing pad 4 in the step of polishing wafers. Therefore, at the time when the groove depth is reduced to a certain value, the polishing pad 4 is judged as having finished its useful life and is replaced by a new one. In other words, the polishing pad 4 is a consumable member and its useful life is determined depending on the remaining depth of the grooves. Since the cost of the polishing pad 4 takes a large proportion of the running cost of the polishing apparatus, there has been a desire for prolonging the useful life of the polishing pad 4 in order to reduce the production cost of wafers.
However, if the grooves are too deep, an amount by which the surface of the polishing pad 4 is projected from bottom surfaces of the grooves is so increased that the surface of the polishing pad 4 is more easily susceptible to elastic deformation. With such elastic deformation, the state of contact between the polishing pad 4 and the wafer is changed, which in turn changes the wafer polishing conditions. Thus, because of a restriction on the groove depth, the useful life of the polishing pad 4 cannot currently be prolonged as desired.
Accordingly, it is an object of the present invention to provide a polishing apparatus and a polishing method, which can polish a material to be polished under a plurality of polishing conditions in a satisfactory manner.
To achieve the above and other objects, the present invention provides a polishing apparatus comprising a plurality of polishing stations for polishing materials to be polished and a plurality of cleaning stations for cleaning the materials being polished. The polishing stations and the cleaning stations are alternately arranged, and an arm is provided for holding the materials to be polished and transferring the materials being polished between the polishing stations and the cleaning stations successively. The arm includes a polishing head for holding the material being polished, each of the cleaning stations comprising a retainer stand on which the material being polished is placed and a cleaning device for cleaning the material being polished while being held by the polishing head, cleaning the material being polished while placed on the retainer stand, and cleaning the polishing head where the material being polished is separated from the polishing head.
With the polishing apparatus thus constructed, when the material being polished is moved between the polishing stations, it is transferred to the cleaning station and cleaned therein before being moved to the next polishing station.
In the cleaning station, the cleaning device cleans the material being polished while held by the polishing head, cleans the material being polished while placed on the retainer stand, and cleans the polishing head where the material being polished is separated from the polishing head. Accordingly, a slurry having intruded into between the material being polished and the polishing head can also be removed. As a result, even in the case of polishing the material to be polished under plural sets of polishing conditions, the risk that slurries having different properties may mix with each other can be reduced, and the material to be polished can be satisfactorily performed under plural sets of polishing conditions.
Also, the present invention provides a polishing method for use in a polishing apparatus comprising a plurality of polishing stations for polishing materials to be polished and an arm for holding the materials being polished and transferring the materials being polished between the polishing stations successively, each of the polishing stations including a platen having a polishing pad affixed to a surface of the platen, the arm including a plurality of polishing heads, each of which holds the material being polished and brings one surface of the material being polished into contact with the polishing pad, the material to be polished being polished by the polishing pad with relative movement between the polishing head and the platen, the polishing method comprising the steps of fitting a spindle for supporting the polishing head in a horizontally rotatable manner into each of fitting portions formed in each of a plurality of spindle housings provided in the arm; rotating the material being polished and the polishing pad relatively while keeping the material being polished and the polishing pad contacted with each other; and adjusting an axial position of the polishing head and changing a position of the polishing head relative to the arm by an adjusting mechanism provided on the spindle, whereby the materials to be polished are each polished while being adjusted in position thereof.
With the polishing method of the present invention, the position of each polishing head is adjusted by the adjusting mechanism provided on the spindle, and the material to be polished is polished in the state where it has been thus adjusted in position. Therefore, the materials to be polished can each be polished under individual appropriate polishing conditions, and the polishing of the material to be polished can be performed with satisfactory results.
Another object of the present invention is to provide a method for conditioning a polishing pad, which can quickly and efficiently remove foreign matter from the polishing pad and can keep the foreign matter from scattering to the surroundings.
To achieve the above object, the present invention provides a method of conditioning a polishing pad for polishing a material to be polished, the method being used with a conditioner comprising a loading eliminating unit for separating foreign matter which is deposited on the polishing pad, from the polishing pad, a cover for covering surroundings of the loading eliminating unit to form a space isolated from the outside between the cover and a surface of the polishing pad, and a sucking unit connected to the space formed between the cover and the surface of the polishing pad for sucking the foreign matter on the polishing pad. The method comprises the steps of separating the foreign matter which is deposited on the polishing pad, from the polishing pad by the loading eliminating unit; sucking the foreign matter on the polishing pad by the sucking unit; and continuing operation of the sucking unit after the step of separating the foreign matter by the loading eliminating unit has been stopped, thereby sucking the foreign matter left on the polishing pad.
With the above method of conditioning the polishing pad, the foreign matter is separated from the polishing pad by the loading eliminating unit, and the separated foreign matter is sucked by the sucking unit and quickly removed from the polishing pad.
Also, since the loading eliminating unit eliminates the loading of the polishing pad in the space formed between the cover and the polishing pad, the separated foreign matter is not scattered to the surroundings.
Further, in parallel to the work of separating the foreign matter from the polishing pad by the loading eliminating unit, the foreign matter on the polishing pad is sucked by the sucking unit and quickly removed from the polishing pad.
Additionally, since the foreign matter left on the polishing pad is removed by the sucking unit even after stopping the operation of the loading eliminating unit, the foreign matter left on the polishing pad is minimized.